1. Field of the Invention
This invention relates generally to systems for gravel-packing one or more production zones of a well, and more particularly, to such systems which permit a liner string and an operating string to be run into the well together, and subsequently permit the gravel-packing of a plurality of production zones to be performed with only a single trip of the operating string into the well.
2. Description of the Prior Art
Unconsolidated formations, particularly those containing loose sands and sandstone strata, present constant problems in well production due to migration of loose sands and degraded sandstone into the well bore as the formation deteriorates under the pressure and flow of fluids therethrough. This migration of particles may eventually clog the flow passages in the production system of the well, and can seriously erode the equipment. In some instances, the clogging of the production system may lead to a complete cessation of flow, or killing of the well.
One method of controlling sand migration into a well bore consists of placing a pack of gravel on the exterior of a perforated or slotted liner or screen which is positioned across an unconsolidated formation to present a barrier to the migrating sand from that formation while still permitting fluid flow. The gravel is carried to the formation in the form of a slurry, the carrier fluid being removed and returned to the surface. The proper size of gravel must be employed to effectively halt sand migration through the pack, the apertures of the liner or screen being gauged so that the gravel will settle out on its exterior, with the slurry fluid carrying the gravel entering the liner or screen from its exterior and being circulated back to the surface.
Prior to effecting the gravel pack, drilling mud and other contaminants may be washed from the well bore, and the formation treated. Commonly employed treatments include acidizing to dissolve formation clays, and injecting stabilizing gels to prevent migration of formation components and formation breakdown prior to packing.
Subsequent to effecting the gravel pack, a reverse-circulation technique may be utilized to remove remaining gravel laden slurry from the operating string utilized to conduct the slurry. With such a reverse-circulation technique, the direction of circulation is reversed and a clean fluid is pumped down the path previously utilized for returning the slurry fluid, and the remaining gravel laden slurry will be forced back up the path originally used to conduct the gravel laden slurry down to the well.
One such prior art system previously used by the assignee of the present invention is disclosed in U.S. Pat. No. 4,273,190 to Baker et al.; U.S. Pat. No. 4,295,524 to Baker et al.; U.S. Pat. No. 4,270,608 to Hendrickson et al.; U.S. Pat. No. 4,369,840 to Szarka et al.; and U.S. Pat. No. 4,296,807 to Hendrickson et al., all assigned to the assignee of the present invention and all hereby incorporated herein by reference. In the system illustrated in the above-referenced patents a liner string is first lowered into the well on a string of drill pipe and set in place in the well. Then, the drill string is disconnected from the liner string and retrieved from the well, and subsequently an operating string of gravel-packing tools is lowered into the well and concentrically into the liner string in order to perform the gravel-packing operation in cooperation with the liner string. Thus, this prior system used by the assignee of the present invention requires two trips of the drill string into the well to perform the gravel-packing operation.
Another prior art system which is designed to accomplish such a gravel-packing operation with only a single trip of the operating string and liner string into the well is shown in U.S. Pat. No. 4,401,158 to Spencer et al. There are, however, several disadvantages of the Spencer et al. system. First, in order to set the liner hanger of the liner string, it is necessary to drop a ball down through the tubing string to seat on an annular seat contained in a liner hanger setting tool of the operating string. It is often difficult, if not impossible, to seat such a ball, if the well bore hole is highly deviated from the vertical. Also, such free-fall or pump-down balls may have to be reverse-circulated out of the well, which is time consuming and again very difficult in highly deviated holes. A second disadvantage of the Spencer et al. system is that return fluid is allowed to flow past screens immediately adjacent unconsolidated zones of the well, as it flows upward through the liner string, and further, this return fluid after it reaches the upper end of the liner string is returned through the well annulus between the operating string and the well casing. Furthermore, when reverse-circulating with the Spencer et al. system, significant amounts of gravel laden slurry may be left in the operating string.
Another prior system for gravel-packing a zone of a well which provides for running the operating string and the liner string into the well together and subsequently performing the gravel-packing operation with only a single trip of the operating string into the well is shown in U.S. Pat. No. 3,710,862 to Young et al.
Thus, while the prior art does include a number of gravel-packing systems, some of which are suitable for gravel-packing multiple zones of a well, and some of which are also suitable for gravel-packing a well with only a single trip of the operating string and liner string into the well, there is still a need for a gravel-packing system suitable for gravel-packing multiple zones of a well with only a single trip of the operating string and liner string into the well, and doing so in a reliable manner. This need is particularly present in the case of highly deviated wells wherein it is extremely difficult to utilize pump-down balls to actuate a liner hanger setting tool of the operating string.